1. Field of the Invention
The present invention generally relates to an optical disk device and a tilt correction method, and more particularly, to an optical disk device, a tilt correction method, a tilt control program, and a recording medium storing the tilt control program which have a function of correcting an angle of a laser so that the laser is always projected perpendicularly to a surface of an optical disk.
2. Description of the Related Art
Conventionally, an objective lens is used for condensing a recording/reproducing laser light and thereby forming a micro optical spot on a track of an optical disk. When an optical axis of the objective lens is inclined with respect to the optical disk, an aberration of the optical spot occurs, possibly causing trouble to the recording/reproduction. To prevent this trouble, an inclination of the objective lens to a recording surface of the optical disk needs to be made as small as possible.
As for the optical disk, CDs, such as a CD-ROM, a CD-R and a CD-RW, are widespread. Remarkably, in recent years, DVDs have begun to emerge with a higher track density and a larger capacity than the CDs. DVDs for reproduction have already been prevalent in general, and DVDs for recording have been put into practical use.
As such optical disk has a higher density, a numerical aperture of the objective lens needs to be made larger; accordingly, the objective lens increasingly needs to be prevented from inclining with respect to the recording surface of the optical disk.
Conventionally, Japanese Patent No. 3114661 (Japanese Laid-Open Patent Application No. 11-39683) and Japanese Laid-Open Patent Application No. 2000-339727 disclose examples of an optical disk device that measures tilts at radial positions of a disk and stores the measured tilts beforehand, and corrects the tilts according to the stored tilts upon a recording/reproduction.
By the way, as shown in FIG. 1, a warp of the optical disk changes according to a rotational velocity. Specifically, as the rotational velocity becomes larger, a flatness of the optical disk becomes improved due to a centrifugal force; accordingly, the warp of the optical disk reduces. According to the reduction of the warp of the optical disk, the tilt of the optical disk decreases. That is, when a measuring of tilts of the optical disk and an actual recording/reproducing of the optical disk are performed at different rotational velocities, the tilts are corrected differently from originally intended. Especially, according to a CLV (Constant Linear Velocity) method, a recording/reproduction of a CD-R/RW or a DVD+R/RW is performed at a high rotational velocity for an inner part thereof, and is performed at a low rotational velocity for an outer part thereof; therefore, a measuring of tilts of the optical disk and an actual recording/reproducing of the optical disk are performed at different rotational velocities. Besides, the recording/reproduction may be performed at various velocities changed optimally according to characteristics of the optical disk; also in this case, a measuring of tilts of the optical disk and an actual recording/reproducing of the optical disk are performed at different rotational velocities. Consequently, the tilts are corrected according to incorrect tilt detection data, causing a problem of deteriorated qualities of the recording/reproduction.
Further, in an optical disk device, upon application of power and during operation, a spindle motor, a seek motor, a laser diode, and elements of a circuit generate heat by energy consumption, and accordingly, the temperature of an optical disk and an optical pickup rises. The temperature rise of the optical disk may possibly change a tilt of the optical disk, and the temperature change in elements of the optical pickup may possibly change an optimal angle of the optical pickup. In this case, when a tilt correction is performed according to a tilt measured beforehand, the tilt correction is performed differently from an originally intended tilt correction, possibly imposing a negative influence on recording/reproducing qualities.
Especially, when continuing a recording to the optical disk for a long time, the temperature rise during the recording may likely change tilt characteristics of the optical disk and the optical pickup, and therefore, a proper tilt correction cannot be performed, which deteriorates a quality of the recording, and may cause a write error. When a write error occurs in a (write-once) optical disk, such as a CD-R or a DVD+R, on which information can be written once, the write error may possibly waste the optical disk.